Color tone reproduction is an important aspect of image processing. Historically, tone correction has been performed on a global basis for a given image. Global tone-correction operations traditionally apply the same correction to all the pixels of an image. In other words, each input value is mapped to one and only one output value.
Global tone correction results in a reasonable correction when the dynamic range of the original image is fairly limited. When the original image has a large dynamic range, it becomes increasingly difficult to perform a global tone correction that will accommodate both shadow and highlight detail.
Consequently, several local tone-correction operations have been proposed for processing images with high dynamic range. A local tone-correction operation typically maps one input value to different output values, depending on the values of the neighboring pixels. This allows for simultaneous shadow and highlight adjustments.
Prior art local-tone correction techniques include manual operations, such as dodging and burning, and automated techniques, such as histogram equalization, piece-wise gamma correction, and Retinex processes. Some of these techniques are quite complex and time consuming. Others do not consistently provide acceptable quality.
An input image represented by a set of input pixel values is color-corrected by locally modifying the input pixel values according to pixel neighborhoods; and determining an output image having a set of output pixel values. Each set of output pixel values equals a non-linear combination of a set of input pixel values and its corresponding set of modified pixel values. Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention.